Quick Answer
The answer is `net.ipv4.ip_forward = 1`. This sysctl parameter must be enabled because it instructs the Linux kernel to forward IP packets between network interfaces—in this case, between eth0 on the 10.0.1.0/24 subnet and eth1 on the 10.0.2.0/24 subnet—allowing the server to act as a router. Without this setting, the kernel drops any packet not destined for the local system, effectively isolating the two networks. On the Linux Foundation Certified System Administrator LFCS exam, this concept tests your understanding of basic routing and kernel parameters; a common trap is confusing this with firewall rules or interface-specific settings. Remember that enabling IP forwarding is a global kernel toggle, not a per-interface configuration. A simple memory tip: think of the “4” in `ipv4` as the number of octets in an IP address, and “forward” as the action needed to pass traffic between subnets—if you want packets to move, you must flip that switch to 1.
This LFCS practice question tests your understanding of networking. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A server needs to forward packets between two networks: 10.0.1.0/24 on eth0 and 10.0.2.0/24 on eth1. Which sysctl parameter must be enabled?
net.ipv4.conf.all.rp_filter = 1
Why wrong: This enables reverse path filtering, not forwarding.
net.ipv4.ip_forward = 1
This enables IP forwarding between interfaces.
net.ipv4.conf.all.accept_source_route = 1
Why wrong: This enables acceptance of source-routed packets, not forwarding.
net.ipv4.conf.all.send_redirects = 0
Why wrong: This disables ICMP redirects, not forwarding.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
net.ipv4.ip_forward = 1
Option B is correct because enabling `net.ipv4.ip_forward = 1` allows the Linux kernel to forward IP packets between network interfaces, which is required for a server to route traffic between the 10.0.1.0/24 and 10.0.2.0/24 subnets. Without this parameter, the kernel drops any packet not destined for the local system, preventing inter-network communication.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
net.ipv4.conf.all.rp_filter = 1
Why it's wrong here
This enables reverse path filtering, not forwarding.
net.ipv4.ip_forward = 1
Why this is correct
This enables IP forwarding between interfaces.
Related concept
Read the scenario before looking for a memorised answer.
net.ipv4.conf.all.accept_source_route = 1
Why it's wrong here
This enables acceptance of source-routed packets, not forwarding.
net.ipv4.conf.all.send_redirects = 0
Why it's wrong here
This disables ICMP redirects, not forwarding.
Common exam traps
The trap here is that candidates often confuse security-related sysctl parameters (like rp_filter or send_redirects) with the actual forwarding control, or mistakenly think that enabling source route acceptance is needed for routing between subnets.
Detailed technical explanation
Under the hood, the `ip_forward` sysctl controls the kernel's `ip_forward` parameter in the IPv4 stack; when set to 1, the kernel's routing table is consulted for each incoming packet, and if a route exists, the packet is forwarded to the appropriate interface. A subtle behavior is that `ip_forward` must be enabled even if using iptables NAT or routing daemons like FRR, and it can be set persistently via `/etc/sysctl.conf` or `sysctl -w net.ipv4.ip_forward=1`. In real-world scenarios, forgetting to enable this parameter is a common cause of routing failures in multi-homed servers or virtual routers.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
What to study next
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FAQ
Networking — This question tests Networking — Read the scenario before looking for a memorised answer..
The correct answer is: net.ipv4.ip_forward = 1 — Option B is correct because enabling `net.ipv4.ip_forward = 1` allows the Linux kernel to forward IP packets between network interfaces, which is required for a server to route traffic between the 10.0.1.0/24 and 10.0.2.0/24 subnets. Without this parameter, the kernel drops any packet not destined for the local system, preventing inter-network communication.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Same concept, more angles
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Variation 1. Which THREE conditions must be met for a Linux system to function as a router between two networks?
hardWhy A: Option A is correct because each interface must have an IP address in its respective subnet for the Linux system to receive packets from that network and forward them to the other. Without an IP address in the subnet, the interface cannot participate in ARP resolution or routing decisions for that network.
Last reviewed: Jun 11, 2026
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